1. Field of the Invention
The invention relates to a process for combusting fuels, in particular waste, in which underfire combustion gas is introduced into the fuel bed from below and overfire combustion gas is introduced into the flue gas flow above the fuel bed.
2. Description of the Related Art
Underfire combustion gas is usually ambient air or ambient air enriched with oxygen that is often warmed before it is introduced into the fuel bed. Oxygen is added to ambient air to produce underfire combustion gas in order to increase combustion intensity, reduce flue gas flow and, in particular, to increase fuel bed temperature and thus improve bottom ash quality.
Changes in fuel composition, i.e. changes also in the heating value of the waste, result inevitably in fluctuations both in the composition and flow of the flue gas during combustion. Fluctuation of the flue gas flow is undesirable because the maximum possible fluctuation must be taken into account in the design and operation of a combustion plant, and this requires a correspondingly uneconomical design of the induced draft fan. The consequence of fluctuation in the oxygen content in the flue gas is that, in the event of a temporary increase in the heating value of the fuel mixture fed, oxygen deficiency occurs and this results in an increase in carbon monoxide (CO) or in other products of incomplete combustion in the flue gas (CO peaks). The plant must therefore be designed for and operated with high levels of excess air if inhomogeneous fuels are used. However, high levels of excess air lead to increased flue gas flow and this requires not only a larger induced draft fan but also larger downstream flue gas cleaning systems.
A temporary increase in the heating value of the fuel mix fed intensifies the negative effect of flue gas fluctuations and leads to premature ignition of the fuel mix, an increased reaction rate, i.e., greater combustion intensity and heat release per unit of time, a decrease in excess air, and an increase in gas temperature. These produce unstable conditions. This effect is particularly pronounced if ambient air enriched with oxygen is used as the underfire combustion gas. The increased oxygen content results in altogether more intensive combustion and more rapid ignition of the fuel after feeding. If, at the same time, the heating value of the fuel mixture feed increases temporarily, oxygen consumption can rise sharply due to the intensive combustion reaction so that, despite oxygen enrichment, oxygen-deficient combustion conditions arise, at least locally. Such instabilities or fluctuations in the combustion process are extremely undesirable because of the effects described above.